Mitotic proliferation of precursor cells, and differentiation and maturation of daughter cells produced by them are necessary for hematopoiesis. Regarding the former, stem cells in several differentiation stages, microenvironments called niches, and growth factors for the respective stem cells are clearly important. Erythropoietin and thrombopoietin are the most important growth factors for hematopoiesis of erythrocytes and hematopoiesis of megakaryocytes and platelets, respectively. Meanwhile, a lot is still unclear regarding differentiation and maturation of the latter. It is fundamental to sort which of the estimated 23,000 human genes to use as a set and which ones not to use for regular cell differentiation and maturation.
In most cells that constitute a living organism, intracellular organelles including the nucleus are maintained even after maturation. On the other hand, some hemocytes such as erythrocytes and platelets lack organelles such as the nucleus.
Thus, differentiation and maturation in adult-type (non-fetal-type) erythropoiesis discard most organelles such as the nucleus and mitochondria, and are completely different processes from regular cell differentiation and maturation. The maturation process in thrombopoiesis is also a dramatic process, and involves enucleation of the nucleus along with release of the whole cytoplasm containing megakaryocyte organelles in the form of several thousand vesicles enveloped by the cell membrane. A hypothesis was proposed at the end of the 1990's that an apoptosis mechanism is actively used in such enucleation, disposal of organelles, and reconstitution of cells to corpuscles, and its verification has become an important research subject. Furthermore, in erythroid differentiation and maturation in the bone marrow, structures called the hematopoietic islands are formed. These are assemblies of erythroblasts formed around macrophages at the center, and thus it is expected that bone marrow macrophages are involved in erythroid differentiation and maturation. Based on the fact that many unprocessed phagocytosed nuclei were left inside the cytoplasm of bone marrow macrophages when DNAase II gene knockout mice prepared in the early 2000's unexpectedly showed embryonic death due to anemia, macrophages were found to be involved in the phagocytosis of enucleation products and such derived from erythroblasts.
On the other hand, when investigating the mechanism of removal of apoptotic cells, the present inventors discovered that when cells receive an apoptosis-inducing stimulus, the S19 ribosomal protein (RP S19), which is a constituent molecule of the ribosome, i.e., the protein synthesis (translation) machinery, becomes cross-linked multimers (dimers or larger oligomers) by the enzymatic activity of cytoplasmic transglutaminase and is released to the outside of the cells [Non-Patent Document 1]. Furthermore, the present inventors elucidated that RP S19 acquires the ability through multimerization to bind to the C5a receptor(s) [Non-Patent Document 2]. C5a receptors were identified as receptors of complement C5a which is a leukocyte chemotactic factor. The present inventors discovered that cells which have received an apoptosis-inducing stimulus start to produce the C5a receptor(s), and that released RP S19 multimers promote apoptosis via the C5a receptor(s) [Non-Patent Document 3]. On the other hand, it was also elucidated that RP S19 multimers recruit monocytes and macrophages by binding to the C5a receptor(s), and make them phagocytose apoptotic cells through local infiltration [Non-Patent Document 4]. That is, RP S19 multimers are thought to achieve smooth phagocytosis of apoptotic cells by synchronizing the execution of apoptosis and recruitment of phagocytic cells.
Regarding hematopoiesis, growth factors for the respective lineages of precursor cells have been identified. For example, recombinants of growth factors such as erythropoictin and thrombopoietin have become available for use, and revolutionary advances are seen in treatments for anemia and thrombocytopenia. However, treatment-resistant hematopoietic failures, which do not respond to growth factors, still remain. Hematopoietic failures can be classified into congenital and acquired, but in either case, establishment of novel therapeutic methods is needed for diseases that are not therapeutically ineffective by growth factors. It is speculated that many hematopoietic failures that do not respond to growth factors are those with abnormalities in the maturation process, i.e., final differentiation of precursor cells such as erythroblasts and megakaryocytes into hemocytes such as erythrocytes and platelets.
It has been reported that RP S19 gene abnormality (heterozygous abnormality) is the cause for 25% of the cases in children's congenital anemia (Diamond-Blackfan anemia) [Non-Patent Documents 5 and 6]. On the other hand, RP S19 is a constituent molecule of ribosome, and this molecule has been reported to be essential for ribosome production (ribosome biogenesis) [Non-Patent Documents 7 and 8]. Previously, it was speculated that involvement of RP S19 abnormality in the development of anemia is mediated by insufficient protein synthesis (translation) ability in erythroblastic cells due to a decrease in the number of ribosomes caused by inadequate ribosome biogenesis. However, direct evidence related to this has not been found, and naturally there are also opposing views of this speculation [Non-Patent Document 9]. Thus, the reason why heterozygous abnormality of the RP S19 gene causes congenital anemia is still unclear.
In the past, factors regulating the final differentiation of the megakaryocytic lineage, or specifically, platelet biogenesis have not been elucidated [Non-Patent Documents 10 and 11].